i have a2007 mercury grand marquis and when I drive the a/c blows out the defroster instead of the vents

if your selector switch is in the correct position for vent/dash divert and it blows out the windshield only then its a electrical diverter door or switch control that is not working, if it blows out the vent/dash but then switches to the windshield when your driving its a vacuum leak to one of your hoses under the dash which are extremely hard to locate not recommended for a novice, seek a mechanic for this preferably your Ford dealer.

The EATC module is at fault. These can be repaired or replaced on eBay pretty cheap. The guy who did mine was eatc-man on ebay. He has a store Ford EATC and more on eBay as well. He offers both repair and replacement. Here is the link http://stores.ebay.com/Ford-EATC-and-more.